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BBXRT Analysis Issue
The distribution of photons in the focal plane can be empirically described
by a pair of error functions, one narrow to fit the central core of the
beam, and one broad to represent a halo, perhaps from geometric scattering.
If P(<q) is the total power contained within a given angular radius q (in
arcminutes) and N1 and N2 are the relative normalizations of the core and
halo components, then
P(<q)= N1 erf (q/s1) + N2 erf (q/s2)
For our best fitting model we find s1 = 1.8 arcmin, s2 = 5.8 arcmin, N1 =
0.65, N2 = 0.35. The resulting half power radius is approximately 1.3
arcmin.
The silicon detectors consist of five separate cylindrically symmetric
detecting elements defined via grooves onto a single silicon block. In
order to prevent the detection in multiple elements of X-rays incident near
the grooves, an X-ray opaque mask was placed in front of the detector. The
mask is shaped to cover the grooves, but occults a width of 0.5mm on each
side of the grooves. Projected onto the sky, this corresponds to a width of
1.5 arcminutes. Because of the spreading of the X-rays in the focal plane
due to the mirror, the mask reduces the effective area by 20% for an
on-axis source, and as much as 50% for a source centered on the
intersection between the central circular mask and one of the radial spurs
of the mask.
Combining the effects of the PSF and the masking, for an on-axis pointing
62% of all focussed events fall in the central detector, 20% fall on the
mask and 17% are distributed among the four outer detectors.
From the point of view of background subtraction, the BBXRT observations
fall into two categories; those performed during orbit day, and those
performed during orbit night. For the orbit night observations, the
background files given in the archive are adequate and correct. To first
order, the orbit night backgrounds come from the internal background of the
detectors described above, scaled directly with the guard rate.
During orbit day, there is an additional background component caused by the
resonant scattering of solar X-rays off the Earth's atmosphere. This leads
to a substantially increased background flux at energies below 1 keV,
particularly a strong 524 eV oxygen line. Channels 20-60 suffer the most
from this geocoronal emission. The exact amount of bright Earth
contamination of the data at low energies is a steep function of Earth
angle as well as guard rate, and this requires a careful selection of
background data. We are still working on an accurate way of generalizing
background subtraction under these circumstances. Thus, the background
spectra in the archive do not include a bright Earth component, and oxygen
and nitrogen lines are visible in the data below 1 keV. Upgrades to the
archive spectra can be expected when a dependable background subtraction
method is finalized.
Above 2 keV the bright Earth contribution becomes negligible and the orbit
night backgrounds work acceptably for the orbit day data.
The Mission Quality files referred to above contain six qualities derived
from the Shuttle telemetry and standard astronomy programs which can help
interpret BBXRT data:
- the Sun angle, or angle between the pointing direction of BBXRT and the Sun;
- the Moon angle (we never detected the Moon and it never interfered
with observations, but this angle is included for completeness);
- the Earth angle, or angle between the pointing direction and the center of
the Earth (note that the Earth subtends a semi-angle of 72 degrees at
the altitude of BBXRT);
- a 1/0 flag indicating whether the nearest Earth limb to the pointing
direction was bright or dark (1=bright);
- a 1/0 flag indicating whether the Shuttle was in orbit day or orbit night
(1=day);
- and the ram angle, or velocity vector angle, between the velocity
vector of the shuttle and the pointing direction.
The non-X-ray background for the BBXRT detectors was very low (see Table 2).
(In units of count s-1 keV-1 field-1, i.e. the sum of data from two
detector elements viewing the same part of the sky.)
While the on-orbit background was a factor of two to three higher than we
anticipated, it should be kept in mind that the Sun was active during the
mission, with two solar flares occurring in the first three days. The
background was found to vary with time, and these variations are strongly
correlated with the guard rate (see below). For energies less than
approximately 4 keV, the diffuse X-ray background dominates the total
non-X-ray background.
At this point it may be worth mentioning some details of the
anticoincidence methods used. The photoelectrically converted signals in
the five detector elements were output to gate leads of jFETs clustered
behind the detector in the cryostat. Also in the cryostat, close to the
FETs, were LEDs which were occasionally flashed on to dissipate the
accumulated charge within the gates. Five separate anticoincidence
techniques were applied: events detected in a given element were compared
against simultaneous events in that or another element (pulse-pulse and
pixel-pixel anticoincidence); against events in any element with deposited
charge larger than that expected from a 14 keV X-ray (Very Large Event
anticoincidence); against firings of the opto-feedback circuits (LEDs); and
against triggering of the guard detector. This guard consists of a pair of
photomultiplier tubes coupled to plastic scintillators, which surround 85
percent of the solid angle around each detector. Events failing these tests
were flagged accordingly and included in the event lists but not in the
accumulations of spectra and light curves.
These anticoincidence techniques were extremely successful in eliminating
background; the best example of how effective they were was our ability to
observe the bright X-ray pulsar Cen X-3 through the South Atlantic Anomaly
with a loss of only 20% of the photons due to anticoincidence flagging.
No X-ray detector previously flown has ever been able to observe sources
during SAA passage.
Using the beam shape described above, we compared data from observations of
Cygnus X-1, Cyg X-2, Cyg X-3 and X0614+09. A number of observations were
used in order to ensure that a consistent result was found. Ratios of
counts in the five pixels of each detector were compared with those
predicted using a series of Monte Carlo simulations of the beam in the
focal plane, for various off-axis angles and azimuths, to determine the
appropriate beam spread and off-axis position. The bottom line is that
there is a mean displacement of 0.822 arcminutes (49 arcsec) between A and
B, and an azimuth of -63.24 degrees. The azimuth of the displacement is
measured from the A3/A4 boundary, and corresponds to a rotation of 63
degrees through B4 towards B1. So, for a source on-axis in A, the
distribution of counts in the outer pixels of B should be maximized in B4,
with a slight excess in B1. Similarly, for a source on-axis in B, the
counts in the outer pixels of A should be maximized in A2, with a slight
excess in A3.
Observations performed off-axis suffer from a measure of vignetting by the
mirror. Because of this effect, it is important to use the response matrix
constructed for the off-axis angle relevant to the observation being
studied. Response matrices covering the full range of off-axis angles are
provided in the BBXRT archive, but it should be noted that although the
energy vignetting is handled correctly in these matrices, no absolute flux
correction for losses due to the mask have been applied.
We have two types of pointing information for the BBXRT observations. The
attitude files (part of the 1993 February release) contain pointing
information for the entire mission; a right ascension, declination and roll
angle every second, in 1950 coordinates, including slewing intervals. This
pointing information was constructed by adding the TAPS (Two-Axis Pointing
System) gimbal angles to the position of the Orbiter -Z-axis. This is only
accurate to maybe 10 or 20 arcminutes, which makes it useful as a guide to
the start and stop times of observations and as a sanity check for what
area of the sky we were looking at, but not directly useful as an absolute
position reference for the calculation of the off-axis angle. A more
accurate position can be determined from the aspect camera solutions.
We have aspect solutions for more than 60% of our observations, and the
mean aspect solution for each pointing is contained in the observation
catalogues accessible via BROWSE or anonymous FTP. For many observations,
no aspect solution exists. The principal reason for this is that the aspect
camera was turned off during orbit day, because the illumination was too
great for it, and during the SAA.
In addition, there were several problems in the first three days with the
processing electronics associated with the aspect camera. These were
eventually eliminated by modifying the settings of a few onboard processing
parameters. There were also intervals when the aspect camera was flooded
with reflected light or Earthglow; sometimes the star field observed was
too sparse, and too few stars were available for an aspect solution, and
for some LMC pointings the field was too rich. Finally, there are of course
no solutions for times when we were Earthblocked or slewing.
The aspect camera boresight was determined from observations of the Crab,
Cygnus X-1, Cyg X-2, Cyg X-3 and X0614+09. A couple of iterations were
performed before settling on the final boresight on which the aspect
solutions contained in the database are calculated. For some bright
sources, the off-axis angle of the observation can be determined using
ray-tracing; these angles are also included in the released database. Where
no formal off-axis angle can be determined, the relative count rates in
each pixel can be used to make an inspired guess. Off-axis vignetting
corrections (i.e., a given off-axis response) are not sensitive to a 1 to 3
arcminute error in the off-axis angle, depending on source count rate.
From examination of data taken when the covers were closed on the BBXRT
instrument, we found evidence for bursts of spurious events that are
probably connected with microphonics. Typically such a burst lasts less
than two milliseconds, and contains up to five events from within a given
detector (usually A) but from several pixels (more often than not A4, A1
and A0). The microphonic events form a major contribution to the low-energy
background for very-low-countrate sources. For brighter sources, the
contribution from microphonics is negligible.
A routine was written to remove such events during the accumulation of
background or low-countrate sources. For observations where the total
countrate (including events rejected by anticoincidence flags) is below 5
counts per second, the accumulated spectra in the database were constructed
using this tool, to ensure no pollution by microphonics. The background
files were also created using this routine.
Table 3 shows the launch and landing times for BBXRT in a range of
different systems:
There are several clocks on board the shuttle that are relevant to this
discussion. BBXRT has its own internal clock, and so do TAPS and the
Orbiter. Of the three, the Orbiter clock is the most reliable; every
60.000000 seconds, this clock emits a so- called MIN pulse. The MET time of
the last-received MIN pulse is embedded in the BBXRT raw telemetry.
Examining the MET and MIN times throughout the mission, it was found that
1 BBXRT minute = 59.99651 +- 0.00001 seconds.
There is no evidence of a day-to-day variation in this value. Thus, the
BBXRT clock loses 5.026 seconds per day, or about 45 seconds over the whole
mission. This estimate agrees well with a pre-launch estimate of 5.04
seconds/day derived using a 0.73-day calibration run.
We also have determined that the TAPS clock runs 9 seconds behind the
Orbiter clock. All derived pointing information includes this correction.
As the the BBXRT observations were so short and BBXRT's strength is
spectroscopy rather than timing analysis, no barycentric corrections have
been applied to the data.
For the 1992 September archival release all the channels have been written
to the spectral files, however it should be noticed that the first few
channels are below the discriminator cutoff and always have zero counts.
(The number of affected channels varies with detector and with the
discriminator setting at the time of the observation.) Also, channel 512 is
the overflow channel and should be ignored in spectral fits. Future
versions of the PHA files may include a QUALITY column to indicate the
'bad' channels.
As a rule of thumb, the central elements had an effective low energy
threshold of around 300 eV. For the outer elements, this threshold was
closer to 500 eV. The central pixels can go lower because of their
intrinsically lower noise and better shielding, surrounded as they are on
all sides by active detector elements.
Deadtime is negligible for all but the brightest sources we observed,
namely the Crab, Cygnus X-2 and (marginally) Cygnus X-3. It is a steep
function of countrate; for the on-axis observation of the Crab (total count
rate including flagged events approx 2000 cts/sec) the deadtime is about
35% in total, while it is less than 10% for the on-axis Cyg X-2 observation
(1150 cts/sec) and less than 5% for Cyg X-3 (278 cts/sec). All other BBXRT
observations have fewer than 200 cts/sec.
The following is a summary of our current knowledge of possible BBXRT
deadtime effects.
There are four reasons why X-rays incident on the BBXRT detectors might not
make it into the telemetry stream:
- The telemetry can only hold 252 events per frame for a total rate
of 2016 events per second. To first order, once the telemetry buffer is
full additional events that occur before the start of the next frame will
be lost. (Actually, the data from each pixel is buffered, so that one event
from each of the ten pixels can be saved after the telemetry buffer is
filled. These saved events are then put into the telemetry buffer at the
start of the next frame.) Note that if the event rates for the pixels are
different, the dead time will be different for each pixel; pixels with
large rates have larger deadtimes.) For the Crab (start MET: 3.055), a plot of event time versus event number
shows glitches every 0.125 seconds, corresponding to the filling of the
telemetry buffer. The glitches have an average dead time of 14.5 msec or
about 12% of the frame duration.
- Events may be flagged with one or more of the five quality flags
(LED, guard, large event, pixel-to-pixel, or pulse pile-up). BBXRT could be
commanded not to put these events into the telemetry, although we never did
this. Spectral data are normally selected to exclude events with any of
these flags set (the archive spectra and lightcurves were accumulated using
only unflagged events).
- Pulse pile-up can also prevent events from getting into the
telemetry. If an event occurs within an interval of about 100 microseconds
of a previous event in the same pixel, the earlier event is flagged and the
second event is not analyzed. The interval was chosen so that unflagged
events are completely unaffected by subsequent events. Many of the flagged
events will only be minimally affected. The exact length of the interval
varies by about 10% from pixel to pixel. If a third event occurs more than
100 microsec after the first event but less than 100 microsec after the
second, the third event will be analyzed and have the pile-up flag set.
For the Crab data referred to above, 13% of events are flagged due to pulse
pile-up, contributing to a flagging ratio of about 20% overall.
- Events in the same pixel that may occur within 10-20 microsec may
not be sensed as different events in the BBXRT electronics. The pulse
heights will then pile up in a nearly linear fashion. For large rates this
can affect the resulting spectrum. To be strict, the convolution of the
spectrum with itself times the fraction of such events should be subtracted
from the spectrum. In practice, considering the count rates we encountered,
we consider this to be unnecessary.
When analyzing data for which a significant signal was detected in several
pixels (i.e. almost every observation) it may be tempting to combine data
from different pixels into a single spectrum. However, this is strongly
discouraged; the resolution of the detectors is so good and the gains are
sufficiently different that smearing of spectral features and distortion of
continuum information can result from such an addition. The best thing to
do (and the technique which is almost universally adopted in BBXRT papers
originating from the LHEA) is to fit the data from the brightest pixels
simultaneously, with chained values for continuum parameters such as photon
index, temperature etc., and separate normalizations.
It should go without saying that for an extended source, one should be
cautious about even this approach to spectral fitting and carefully treat
the pixels separately before attempting to combine data in a joint fit.
The BBXRT detector spectral response matrices have been painstakingly
constructed based on data from pre-launch, in-flight and post-flight
calibrations. Before launch, measurements of the effective area of the
mirrors and the detector efficiencies were made at GSFC. In flight,
spectral calibrations were performed by moving an internal Fe 55 source
into the field of view and observing the Mn Ka and Kb lines. The Crab
Nebula was observed on several occasions.
More detailed modeling of partial charge in the detectors, and adjustments
for the gold M edge in the mirrors and Al and Si edges in the detectors,
have been performed since the flight of BBXRT, using further ground
calibration data obtained at GSFC and the National Institute for Standards
and Technology in 1991 and 1992. Using the most recent response matrix,
the systematic residuals from a power law fit to data from the Crab Nebula
show no features larger than 5% between 0.5-8 keV and 10% between
8-12 keV, and no narrow features in the spectrum with an equivalent
width >12 eV. A 10% residual uncertainty still persists at ~0.4-0.5 keV.
A full description of the BBXRT calibration process can be found in
Weaver et. al, 1994. ApJSupp (in press).
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Page authors: Lorella Angelini Jesse Allen
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